U.S. Vice President Joe Biden apologized to Prime Minister Shinzo Abe on Wednesday for “causing trouble,” after documents released last week detailed alleged spying by the U.S. National Security Agency on the government in Tokyo, a top Japanese official said.

Chief Cabinet Secretary Yoshihide Suga said the phone conversation between Biden and Abe came about at Washington’s request.

Suga declined to comment on whether Biden admitted the U.S. had spied on Japanese officials and companies over a period that started in 2006, as alleged in documents released last week by anti-secrecy group WiliLeaks.

He quoted the vice president as saying that the U.S. is not conducting any “activities that would damage trust in the Japan-U.S. alliance,” based on an order issued last year by U.S. President Barack Obama.

That remark suggests the NSA could have been spying on Tokyo before Obama issued the order.

Suga said he would not divulge further details about the content of the conversation.

“We’d like to refrain from commenting on details, given the nature of the issue,” Suga said at a daily news briefing.

On Friday, WikiLeaks released a trove of documents titled “Target Tokyo” that appeared to show the NSA had been spying on the Japanese government as well as major Japanese companies. A document of the purported top 35 targets for telephone intercepts included the executive secretary of the chief Cabinet secretary’s office and a phone number described as “Government VIP Line.”

Suga said Abe told Biden that if Japanese parties had indeed been targeted by the NSA as alleged, it could seriously damage trust between the allied countries and he “would have no choice but to express grave concern.”

Abe also demanded that the U.S. investigate the alleged spying and report its findings to Tokyo, Suga quoted the prime minister as saying.

Abe and Biden then spoke about the importance of the bilateral military alliance and agreed to “continue discussion” to “take proper actions,” Suga said, without elaborating.

Analysis of Abe’s comments to Biden suggests an attempt to maintain his public image as a tough but diplomatic leader.

Tokyo is often criticized for being weak-kneed against the U.S., as the bilateral alliance is a cornerstone of its diplomatic posture.

Abe’s public reaction to the WikiLeaks allegations was subdued compared with that of German Chancellor Angela Merkel. The German leader issued a furious protest against Washington after German magazine Der Spiegel reported in 2013 that the NSA was tapping her cellphone, citing documents provided by WikiLeaks.